1. Field of the Invention
The present invention relates to a shape measuring apparatus configured to measure a surface shape (figure or profile) of a measurement target, a shape measuring method configured to measure a surface shape of a measurement target, and an exposure apparatus including the shape measuring apparatus.
2. Description of the Related Art
As the background art of a shape measuring apparatus and an exposure apparatus including the shape measuring apparatus, the following description is given of, in particular, an example of a semiconductor exposure apparatus which requires severe measurement accuracy of a surface shape.
When a semiconductor device or a liquid crystal display device is produced by using the photolithography technique, a projection exposure apparatus is used in which a circuit pattern drawn on a reticle is projected for exposure to a wafer by a projection optical system.
In the projection exposure apparatus, with an increase in integration of semiconductor devices, the circuit pattern drawn on the reticle is demanded to be projected for exposure to the wafer at higher resolving power. A minimum dimension (or finest resolution) transferable by the projection exposure apparatus is directly proportional to the wavelength of light used for the exposure and is inversely proportional to the numerical aperture (NA) of the projection optical system. Accordingly, the shorter the wavelength of the exposure light, the higher is the resolving power. For that reason, light sources having shorter wavelengths, such as a KrF excimer laser (with a wavelength of about 248 nm) and an ArF excimer laser (with a wavelength of about 193 nm), have been recently used. Also, practical use of liquid immersion exposure has been progressed. In addition, further enlargement of an exposure area has been demanded.
To achieve those demands, a scanner has been mainly used instead of a step-and-repeat exposure apparatus (also called a “stepper”) in which a substantially square exposure area is exposed to a wafer at a time with a reduction (scale-down). The scanner is a step-and-scan exposure apparatus in which an exposure area is formed as a rectangular slit and a reticle and a wafer are relatively scanned at a high speed to perform exposure of a large region with high accuracy.
In the scanner, before a predetermined position of the wafer reaches the exposure slit area, a surface position of the wafer at the predetermined position is measured by a surface position measuring unit (focus control sensor) with a light oblique incidence system. Based on the measured surface position of the wafer, a correction for aligning (registering) the wafer surface with an optimum imaging surface is performed when the predetermined position of the wafer is exposed.
Particularly, plural measurement points are set in the exposure slit area along the lengthwise direction of the exposure slit (i.e., along a direction perpendicular to the scanning direction) to measure not only the height of the wafer surface position (i.e., “focus”), but also the inclination of the wafer surface (i.e., “tilt”). As methods of measuring the focus and the tilt, there are known a method using an optical sensor (see Japanese Patent Laid-Open No. 6-260391 and U.S. Pat. No. 6,249,351), a method using a gas gauge sensor (see Pamphlet of International Publication WO2005/022082), and a method using a capacitance sensor.
In recent years, however, with the use of a shorter wavelength of the exposure light and a higher NA value of the projection optical system, the focal depth has become so extremely small that it is more difficult to realize satisfactory accuracy in aligning the exposed wafer surface with the optimum imaging surface, which is called focusing accuracy. In other words, some factors have become not negligible which include, particularly, the influence of a pattern on the wafer and errors in measurement of the surface position measuring apparatus, which are attributable to unevenness in the thickness of a resist coated on the wafer.
For example, the unevenness in the thickness of the resist causes a level difference, which is serious for the focus measurement although it is smaller than the focal depth, near a peripheral circuit pattern and a scribe line. Therefore, an inclination angle of the resist surface is increased to such an extent that reflected light, which is detected by the surface position measuring apparatus, is deviated from an angle of specular reflection due to reflection and/or refraction. Further, a difference in roughness/fineness of the pattern on the wafer generates a difference in reflectivity between a fine pattern area and a rough pattern area. Thus, because of changes in the angle of reflection and in the intensity of the reflected light which are detected by the surface position measuring apparatus, the waveform of a signal resulting from detecting the reflected light becomes asymmetric and a measurement error is caused.
FIG. 18 is a schematic view illustrating the case where measurement light MM is illuminated to a wafer SB having a difference in reflectivity when the optical sensor disclosed in Japanese Patent Laid-Open No. 6-260391 is used. In the illustrated case, the measurement light MM is inclined by an angle A relative to a boundary line between two areas differing in reflectivity, and the measurement is performed in a direction denoted by α′. FIG. 19 plots intensity distributions of reflected light in three cross-sections spaced from each other in a direction denoted by β′, i.e., an AA′ section, a BB′ section, and a CC′ section. As seen from FIG. 19, the reflected light has good symmetry in the AA′ section and the CC′ section, while the reflected light has an asymmetrical profile in the BB′ section which includes both the areas differing in reflectivity. Such an asymmetrical profile shifts the barycenter in distribution of the reflected light and causes a measurement error. Accordingly, the wafer surface cannot be measured with high accuracy and a large defocus is generated, thus resulting in a chip failure.
FIG. 15 illustrates a shape measuring apparatus disclosed in U.S. Pat. No. 6,249,351 in which light is obliquely illuminated to a substrate and a shape of the substrate is measured based on a resulting interference signal. The disclosed shape measuring apparatus includes a light source 101, a lens 103, a beam splitter 105, a reference mirror 130, a driving mechanism 397, a beam combiner 170 formed of a grating, lenses 171 and 173, and an image pickup element 190. Wide-band light (white light) from the light source 101 is introduced to the beam splitter 105 through the lens 103 and is divided into reference light and measurement light. The reference light is reflected by the reference mirror 130 and the measurement light is reflected by a wafer 360 which is a sample. Those reflected lights are combined together by the beam combiner 170 formed of the grating. The reference light and the measurement light interfere with each other, and resulting interference light is introduced to the image pickup element 190 through the lenses 171 and 173.
The disclosed shape measuring apparatus also has the problem that the surface shape is erroneously measured by the influence of a circuit pattern on the wafer 360. That problem will be described in detail with reference to FIGS. 16, 17A and 17B.
FIG. 16 plots the intensity of the so-called “white interference signal” obtained in the shape measuring apparatus of FIG. 15 when the wafer 360 is moved by the driving mechanism 397 in a direction perpendicular to the wafer surface. A signal in Case 1 in FIG. 16 represents the case of measuring the wafer 360 having a structure in which no pattern is formed on the wafer 360 and only a resist is coated thereon as shown in FIG. 17A. On the other hand, a signal in Case 2 in FIG. 16 represents the case of measuring the wafer 360 having a more general structure in which a pattern is formed on the wafer 360 and a resist is coated the pattern as shown in FIG. 17B.
Looking at FIG. 16, in comparison with the signal in Case 1, the signal in Case 2 is affected by the pattern on the wafer 360 such that the interference signal is partly distorted. The distortion of the interference signal is attributable to a specific system of shape measuring apparatus of FIG. 15 in which, as shown in FIG. 17B, light is obliquely illuminated to the surface of the wafer 360 and the reflected light from the wafer surface is received. More specifically, when the wafer 360 is scanned in the direction perpendicular to the surface of the wafer 360, the position on the wafer 360 illuminated by the measurement light is shifted and a measurement point on the wafer 360 is changed. Therefore, the intensity of the reflected light is changed by the influence of the circuit pattern on the wafer and a correct interference signal cannot be obtained. Rays of light shown in FIGS. 17A and 17B represent only the light that passes the resist surface and is reflected by the wafer surface. In Case 2 of FIG. 16, because the reflectivity is partly increased, a peak position of the white interference signal is changed and an error is eventually generated in a value obtained by measuring the shape profile of the wafer.
Further, the method using a gas gauge sensor as described in Pamphlet of International Publication WO2005/022082 has the problem that minute particles mixed in gas are sprayed toward a wafer. As another problem, that method cannot be used in an exposure apparatus operated in vacuum, e.g., an EUV (Extreme Ultraviolet) exposure apparatus, because a vacuum level is deteriorated by the gas.